Antiskid system

ABSTRACT

A means to regulate hydraulic system pressure based upon a function of the work produced by one set of actuators and having a device to monitor the pressure from said means to another set of actuators as a function of the comparison of their work required with the work done by the one set of actuators.

United StatesPatent [72] lnventor Lester J. Larsen South Bend, Ind. 211Appl. No. 756,259 [22] Filed Aug. 29, 1968 [45] Patented Apr. 27, 1971[73] Assignee The Bendix Corporation [54] ANTI'SKID SYSTEM 5 Claim, 10Drawing Figs.

[52] US. Cl 303/21, 188/ 18], 303/6, 303/22 [51] Int. Cl B60t 8/08, B60t8/26 [50] Field oiSearch 303/6, 6 (C),2l,22,24;183/181 [56] ReferencesCited UNITED STATES PATENTS 3,995,522 6/1963 Packeret a1. 303/21 H I I200 3,479,094 1 1/ 1969 Chovings 303/21X 3,486,800 12/1969 Ayers 303/211,926,296 9/ 1 933 Merchie 303/21X 2,088,185 7/1937 Borde 303/212,181,161 11/1939 Wolf 303/21UX 3,093,422 6/1963 Packer et a1 303/213,443,842 5/1969 Pier 303/22X Pn'rfiary Examiner-Mi1ton BuchlerAssistant Examiner-John J. McLaughlin Attomeys-Richard G. Geib andPlante, Arens, Hartz and OBrien ABSTRACT: A means to regulate hydraulicsystem pressure based upon a function of the work produced by one set ofactuators and having a device to monitor the pressure from said means toanother set of actuators as a function of the comparison of their workrequired with the work done by the one set of actuators.

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WESSMME? ATTORNEY PATENTEU M27197! 3576350 Y SHEET UF 6 /6 m we M2INVENTOR LESTER J- LABSEN ATTORNEY PATENTED m Re zsn 3578350 sum 5 or 6v m I .96 -32lfl$933d INVENTOR LESTER J. LARSEN BY i ATTORNEY ANTI-SKIDSYSTEM SUMMARY In the development of antiskid systems, it has been shownthat maximum benefits of (l) retained steerability, (2) retaineddirectional stability, and (3) reduced stopping distances as compared tolocked wheel stopping distances are obtainable by systems which affectthe braking of all four wheels individually in response to a sensedperformance characteristic of the respective wheel which is beingcontrolled. Such a system is very expensive in that it requires fourwheel characteristics sensors, four brake pressure modulating devices,and four channels of control logic. In a first attempt to reduce thecost and complexity of such a system, it has been shown to be practicalto combine the control of the two rear-axle mounted wheels into a singlecontrol channel using a single brake pressure modulator while continuingto control the front wheels individually. Initial efforts in thisdirection also involved the use of a single sensor mounted on thevehicle drive shaft. These efforts failed because the derived signalswere not truly indicative of actual wheel performance because of thepresence of the differential gear mechanism.

A successful rear-axle antiskid control has been developed. This systeminvolves the use of individual wheel sensors and a single logic andcontrol channel which selects that wheel indication which is provided bythe wheel operating at the lowest speed. An antiskid control of thischaracter, in effect, is regulating the braking effort in proportion tothe lowest coefi'rcient road surface contacted by either one of thewheels of the respective axle. Such systems are referred to as selectlow systems. Due to factors which need not be explained in relation tothe current invention, such systems are capable of showing improvedstopping distances under most circumstances.

OBJECTS In the present invention the inventor has attempted toaccomplish a further reduction in the number of components required forthe system while, at the same time, providing performance which, undermost operating conditions, shows measurable gains in all three of thecategories previously enumerated. In this system braking pressuresupplied to all four wheels is controlled by an antiskid pressuremodulator in accordance with front wheel behavior measured by individualwheel sensors mounted on each of the front wheels which transmit theirindications to a logic and control channel operating on a select low" or"select high basis. The pressure to the rear brakes is further modifiedby operation of a load sensing proportioning valve constructed andarranged substantially in accordance with that shown in US. Pat. No.3,362,758 issued Jan. 9, 1968 by the common assignee.

The load sensing proportioning valve is designed to reduce pressure tothe rear brakes as a function of the distance between the rear axlehousing and the vehicle frame or body. This distance changes in responseboth to variations in static load and braking deceleration due to thewell-known weight shift characteristic which results from the fact thatthe vehicle center of gravity is always positioned substantially higherthan the road surface.

DRAWING DESCRIPTION One embodiment of my invention is illustrated in theaccompanying drawings in which:

FIG. 1 is a schematic system diagram;

FIG. 2 is a longitudinal sectional view of an antiskid brake pressuremodulator of a type suitable for use in the subject system;

FIG. 2a is a partial cross section of the modulator piston of FIG. 2showing a tandem cylinder for controlling a dual brake system;

FIG. 3 is a longitudinal sectional view of a brake proportioning valveinstalled in a manner suitable to operation in the subject system;

FIG. 4 is a graph of the relationship between rear brake pressure andfront brake pressure as determined by the operation of the load sensingproportioning valve;

FIG. 5 is a longitudinal section through a front disc brake installationshowing a wheel characteristic sensor installed for the purpose ofmeasuring the required wheel performance characteristics;

FIG. 6 is a lateral cross section view of the sensor and brake shown inFIG. 5;

FIG. 7 is an axial cross section of the sensor per se; and

FIGS. 8 and 9 are reproductions of oscillograph recordings of actualvehicle performance under control of the system of this invention.

DETAILED DESCRIPTION cylinder 24. A conduit 26 conducts brake fluid toan antiskid modulator designated generally by the numeral 28. A conduitsystem designated only by the numeral 30 extends from the outlet port ofthe pressure modulator 28 to each of the front disc brakes and to theinlet port of the load sensing proportioning valve which is designatedgenerally by the numeral 32. If the barking system is of dual naturethen there would be two conduits from the master cylinder to themodulator and one conduit therefrom to the front brakes and anotherconduit therefrom to the load sensing proportioning valve. The loadsensing proportioning valve is shown mounted on the rear axle housing 34and is provided with an operating lever 36 having one end a notch 38 inwhich one end of an extension spring 40 is secured. A bracket 42extending from a portion of the vehicle frame 44 provides a point ofattachment at 46 for the other end of the extension spring 40. A conduit48 extends from the outlet port of the load sensing pressure modulatingvalve to the wheel cylinders 50 and 52, respectively, of the drum brakes14 and 16. Since the power brake master cylinder unit 22-24 is ofcompletely conventional configuration, I do not feel it necessary todescribe or show its interior construction.

Referring to FIG. 2, the antiskid brake pressure modulator 28, whichdoes not in itself comprise part of my invention, consists of a vacuumchamber housing made up of two cupshaped stampings 54 and 56 whose rimsare clamped together in sealed relationship with the flange 58 of aconventional rolling diaphragm 60. The rolling diaphragm 60 is mountedon a piston 62 and the diaphragm and piston divides the housing into twochambers, which I will designate as a vacuum chamber 64 and a vacuum-airchamber 66. A very powerful spring 68 is located in the vacuum chamber64 and urges the piston 62 to the right-hand end of the unit. Ahydraulic cylinder 70 is mounted concentrically with the cup member 56and extends into the vacuum chamber forming a guide for the right-handportion of the vacuum piston 62. A tube 72 is secured in the cup member54 to serve as a guide for the lefthand end of the vacuum piston 62. Thehydraulic cylinder 70 is formed with an interior bore 74 in which isseated a hydraulic piston 76 which projects through a seal 78 in theleftward end of the cylinder 70 and engages the vacuum piston 62. Thehydraulic cylinder 70 is formed at its right end with an inlet portwhich receives the conduit 26 leading from the master cylinder and witha port 82 which receives the conduit 30 leading to the front brakes andthe load sensing proportioning valve. A check valve 84 is located in theport and, in the normal or released position of the pressure modulator,a pin 86, forming a rightward extension of the hydraulic piston 76,holds this check valve away from its seat. In this condition, it will beobserved that brake fluid under pressure can flow freely from conduit 26to the brakes through conduit 30.

A vacuum connection 88 is connected to a conduit 90 which is, in turn,connected to the intake manifold 92 of the vehicle engine 94, as seen inH6. 1. The vacuum connection 88 is formed as a plastic molding in theshape of a T, one branch 89 of which enters the vacuum chamber 64. Thethird arm of the T 91 is connected by a conduit 94 to a normally opensolenoid valve, designated generally by the numeral 96, mounted on theright-hand end of the cup member 56. A rubber disc check valve 98 ispositioned in the vacuum inlet chamber in such a way that vacuum istrapped in anns 90 and 92 even though the engine stops and atmosphericpressure is established at vacuum connection 88. A normally closedsolenoid valve 100 is also mounted on the right-hand end of cup 56 andis provided with an air inlet 102 which admits air to the vacuum-airchamber 66 through a passage 104 when the solenoid valve 100 isenergized. The normally open solenoid valve 96 permits vacuum tocommunicate through a passage 106 with the vacuum-air chamber 66whenever it is not energized, and, when energized, it interrupts theconnection between the vacuum conduit 104 and the passage 106,preventing increase of vacuum in the chamber 66.

in FIG. 2a the cylinder 70a is for a dual brake system. It has separateinlets 80a and 80b, separate outlets 82a and 82b and separate checkvalves 84a and 8412. Two pistons 76a and 76b control valves 84a and 84bby pins 86a and 86b. These pistons extend beyond the cylinder intoabutment with the vacuum piston 62 so that the valves 84a and 84b wouldbe controllable by segregated master cylinder pressures and by thepiston 62.

Referring now to FIG. 3, the load sensing proportioning valve 32comprises a cast housing 108 having a central bore 110 having a closedend 112 and an open end which is partially closed by a threaded plug114. The plug 114 is formed with a concentric longitudinal bore 116 ofsubstantially smaller diameter than the housing bore 110. A steppedpiston 118 has its large end 120 positioned in fitted relationship tothe housing bore 110 and its smaller end 122 extends in fittedrelationship through the passage 116 in the plug 114. The clearancebetween the small end 122 of the piston 118 and the bore 116 of the plug114 is protected against the entrance of contaminants by a rubber boot124. Escape of pressure fluid through this same clearance is preventedby a cup seal 126 installed in a suitable annular groove formed in thepiston. The housing bore 110 constitutes a chamber having an upperportion 128 defined by a portion of bore 110, by portions of steppedpiston 118, and the inner end face of the nut 114. This chamber 128 isprovided with an inlet port 130 to which is connected the conduitleading from the antiskid brake pressure modulator. The chamber formedby the housing bore 110 also has a lower portion 132 defined by aportion of the walls of the bore 110, the lower face of the piston 118and the closed end 112 of the housing. An outlet port 134 communicateswith this chamber and is connected to the conduit 48 which leads to therear brake wheel cylinders 50 and 52. A cup seal 136 is seated in asuitable groove formed in the large end 120 of the piston 118 and isoriented to seal against passage of fluid from the upper chamber 128 tolower chamber 132. The construction of this seal is such that passage offluid from chamber 132 to chamber 128 will not be inhibited if apressure difference exists in that direction. The stepped piston 118 isformed with a longitudinal passage 138 extending from the lower face ofthe large end 120. A threaded bushing 140 is screwed into suitablethreads formed in the lower end of this passage. The upper end of thisbushing constitutes a valve seat upon which is seated a valve ball 142.A small spring 144 serves to urge this ball on to its seat.

A pin 146 is slidably disposed in the bore of the threaded bushing 140and is of such length that when the piston 118 is urged to its lowermostposition in contact with the end wall 112 of the housing bore 110, thevalve ball 142 will be lifted from its seat. The pin 144 is ofnoncircular cross section so as to from a fluid passage whereby fluidmay flow through a lateral passage 148 into the passage 138 past thecheck ball 142 through the bore in the bushing 140 to the chamber 132.

The cast housing 108 is formed with a bracket 150 to which the lever 36is swingably secured on a pin 152. The lever 36 rests in contact withthe projecting end 122 of the stepped piston 118 and with the spring 40installed as described in relation to FIG. 1, the force of the spring isexerted on the piston 118 to urge it to its lowermost position incontact with housing end wall 112 and hold it there at somepredetermined force. It will be observed, however, that as the distancebetween the axle housing 34 and vehicle frame 44 varies, the spring 44will be either stretched or relaxed in such a way as to increase ordecrease this predetermined force.

Referring to FIGS. 5,6, and 7, installation of suitable wheel operatingcharacteristic sensors is shown although this does not constitute a partof my invention per se. In these FIGS. is shown a front wheel spindle154 upon which a wheel hub 156 is pivoted on suitable bearings 157. Abrake disc 158 is secured to the wheel hub 156 by suitable studs 160.Pivoted on the wheel hub 156 and secured thereto by machine screws 162is a heat insulating ring 164 formed with an annular groove 166 in whichis seated a friction drive ring 168 of suitable elastomeric material. Abracket 170 (see FIG. 7) is secured to a portion of the wheel spindle154 by bolts 172 and the sensor, which I will designate generally by thenumeral 174, is pivotably mounted to said bracket by a pin 176. Thesensor 174 is provided with a drive roller 178 and a mousetrap spring180 resiliently urges the sensor to swing about its pivot pin 176 tobring the drive roller 178 into driving contact relationship with thefriction element 168. Thus, it will be seen that the sensor drive roller178 will be driven at a speed in a fixed relationship to the speed ofrotation of the vehicle wheel. Referring specifically to the sectionalview of HO. 7, the sensor 174 comprises a generally circular casthousing 182. A toothed wheel 184 is enclosed within the cavity of saidhousing by a cover plate 186 secured by suitable screws 188. The toothedwheel 184 is mounted on a shaft 190 carried in suitable bearings 192 ina hub 194 which is formed integrally with the housing 182. The frictiondrive roller 178 is secured to the outer end of the shaft 190, Amagnetic proximity sensor 196 is threaded into a lateral opening in thehousing 186 in such a way that its magnetic pole portion 198 ispositioned in close proximity to the ends of the teeth on the toothedwheel 184. The sensor 196 is provided with a permanent magnetic elementand an induction coil of suitable characteristics and the toothed wheelhas teeth of correct shape and proportion to generate electricalimpulses upon the passage of each tooth past the pole face.

Referring back to FIG. 1, the sensors 174 are shown electricallyconnected by cables 200 to an electronic unit designated generally bythe numeral 202. This unit is provided with electric energy from thevehicle battery 204. A switch 206 positioned so as to be actuated byinitial movement of the brake pedal 18 is also electrically connected tothe unit 202 so as to energize it upon initiation of braking and make itready to perform the antiskid function as required. The unit 202includes logic and control elements which, in detail, are not a part ofmy invention but which may be of the type shown in US. Pat. No.3,494,671. The signals received from the wheel sensors 174 are connectedinto control signals and electrical energy is transmitted throughelectric wires 208 and 210 to the normally open and normally closedsolenoid valves 96 and 100, respectively, for the purpose of making theantiskid brake pressure modulators function in a manner which will bedescribed subsequently. As I have described them, the wheel sensors 174constitute pulse generators which produce an alternating current at afrequency proportional to wheel speed. This pulsing, or alternating,current is passed through a frequency converter which produces a DCvoltage proportional to wheel speed. The voltages from the two sensorsare compared and the lower voltage is passed through a discriminator tothe control system. Since this voltage represents wheel frequency andthe antiskid system is best controlled in relation to wheelacceleration, both positive and negative, this signal voltage isprocessed to produce a first derivative which is proportional to theacceleration of the wheel. It is well known in antiskid technology thatthe maximum rate of vehicle deceleration is approximately lg, so ifwheel deceleration exceeds a value slightly higher than lg, it isobvious that this wheel is approaching a lockup condition. This can beestablished as the first set point at which the control system signalsthe pressure modulator to reduce braking pressure by closing valve 84and retracting piston 76, or valves 84a and 84b and pistons 76a and 7611as the case may be. As braking pressure is reduced, the rate of wheeldeceleration reduces and at some point the wheel will begin toaccelerate back to synchronous speed. Some level of acceleration isestablished as a second set point and, upon receiving this intelligencefrom the wheel sensor, the control system signals the pressure modulatorto stop reducing pressure and instigates a slow rate of pressureincrease by displacing piston 76 or 76a and 76b, with spring 68 upon areduction of pressure in vacuum-air chamber 66. At some point later intime, if the first deceleration set point has not been exceeded again, ahigh rate of pressure rise will be instigated. Shortly thereafter, thesaid first deceleration set point should be exceeded again and thecontrol cycle will repeat itself.

It should be fairly obvious to most persons familiar with conventionalautomobile brake performance that controlling the brake pressure to therear brakes, in the same manner as l have just described with relationto the front brakes, would not prevent the rear brakes from locking wasa result of normal weight shift when the vehicle is being rapidlydecelerated on a good road surface. For this reason, as an importantelement of my invention, I have incorporated into the system the loadproportioning valve 32, which in itself is not novel in the brakeindustry. In the previously identified Pat. application relating to sucha valve, it has been explained that the valve can be built toautomatically vary the relationship of front and rear brake pressure tocompensate both for static load variations such as the addition orremoval of extra passengers or luggage, and also for variations of rearwheel load caused by the previously mentioned weight shift due to theefiects of inertia upon the mass of the vehicle while braking. However,in an otherwise conventional automobile equipped with a load sensingproportioning valve for the rear brakes, if the driver attempts to makean emergency stop and applies sufficient energization to the-brakesystem to cause the front brakes to lock, the rear brakes will alsoreceive sufiicient pressure to enable them to be locked. However, in avehicle equipped in accordance with my invention, since the pressure tothe front brakes is regulated by the antiskid mechanism in such a waythat locking of a front wheel is impossible, it can be understood thatthe functioning of the load sensing pressure modulator willsimultaneously prevent the locking of either or both of the rear wheels.

FIGS. 8 and 9 represent tracings of actual oscillograph recordings ofthe behavior of a vehicle equipped with an antiskid system built inaccordance with my invention. FIG. 8 was recorded during a stop onsmooth, dry asphalt, constituting a relatively high frictioncoefficientsurface. FIG. 9 was recorded during a stop on a surface made up of loosegravel and cinders and, in view of the deceleration attained, aboutftjsecfi, should be considered a medium coefficient surface. Thedifference in cycling behavior illustrates the manner in which thesystem adapts to road surfaces having different characteristics.

Referring to FIG. 8, Trace 01 represents rear brake pressure, that is,pressure in the conduit 48/ Trace 02 represents front brake pressure, orthe brake pressure in the conduit 30. Trace 03 represents vehicledeceleration. Trace 04 represents the speed of the left front wheel.Trace 05 represents the speed of the right front wheel. Trace 06represents the speed of the left rear wheel. It should be noted that theTraces 01 and 02 both begin at zero brake pressure; and that Traces 04,5, and 6 end at zero wheel speed. They are shown spaced for clarity.

With this in mind, the operation of my brake system is clearly presentedto show the system of FIG. 1 to be working from the beginning of brakeapplication, or point A of this FIG. Moreparticularly, from point A topoint B the following has occurred:

At about 900 p.s.i. front brake pressure and 400 p.s.i. rear brakepressure the modulator 28 has been activated to close check valve 84 andlock out further supply of pressure from master cylinder 24. In doingthis, the modulator also relieves pressures by increasing displacementin that piston 76 follows up the movement of piston 62 to return thefront brake pressure to approximately 500 p.s.i. whereupon atmosphericair is cut off from passage 104 by solenoid valve 100.

During this the rear brake proportioning valve 32 has sensed a weightshift via the spring 40 to close the valve ball 142 at approximately 150p.s.i. to proportion the rear brake pressure as a function of the inletand outlet areas of stepped piston I18. As seen in FIG. 8 the rear brakepressure climbs to 400 p.s.i. and then is relieved by the modulatorincreasing the displacement in thesystem between it and theproportioning valve.

At the time of closure of the solenoid valve the front brake pressurehas been reduced by 400 p.s.i. and the rear brake pressure by I00 p.s.i.and wheel lock has not occurred while vehicle deceleration, asrepresented by Trace 03 has reached a maximum value permitted by theelectronic unit 202.

Unit 202 which senses the result of these braking pressure changes nowreopens the solenoid valve 96 so that spring 68 will move piston 76 tobegin redevelopment of front brake pressure to 900 p.s.i., as the rearbrake proportioning valve maintains approximately 300 p.s.i. in the rearbrakes called for by the center of gravity shift of the vehicle as amaximum rear brake pressure for maximum rear brake effectiveness.

When the front brake pressure reaches 900 psi. again the sensors 174tell the unit 202 to apply atmosphere to chamber 66 of modulator 28 andthe brake system pressure is again reduced and reestablished, as before.

Therefore, braking was initiated at point A and the stop was completedat point B. The antiskid control system cycled three times during thisperiod, which represents an elapsed time of 2 /4 seconds. In thisparticular case, the left front wheel appeared to be the controllingwheel, and it will be noted that while it approached the lockedcondition briefly at the beginning of the last cycle, no actual lockupoccurred during the entire stop. A high average deceleration wasmaintained during the entire stop and in all testing to date stoppingdistances have been consistently lower than locked wheel stoppingdistances of the same vehicle on the same surfaces.

Referring to FIG. 9, the cycling rate increased to about 10cycles/second and the pressure and velocity excursions weresubstantially reduced.v In this FIG. the trace designations are the sameas in FIG. 8 with the suffix a applied, and the trace for rear brakepressure is omitted.

Iclaim:

1. In a wheeled vehicle with front and rear wheels having a pressureoperated brake system, said brake system comprising:

a booster type brake unit;

separate actuation means for each wheel of said vehicle,

said actuation means being responsive to pressure from said booster typebrake unit;

sensor means for detecting the rotational velocity of at least one wheelof said wheeled vehicle and generating a sensor output signal for eachdetected wheel;

control means for converting said sensor output signals into a controlsignal; modulation means interposed between said booster type brake unitand said separate actuation means, said modulation means being operatedby said control signal to reduce brake pressure and prevent wheelskidding; proportioning means interposed between said modulation meansand said separation actuation means for the rear wheels, saidproportioning means automatically redistributing braking effort betweenthe front and rear wheels as a function of load shift; and

said modulation means having valve means to operate said modulationmeans in response to said control signal, said control signal operatingsaid valve means to provide multiple build rates in said brake pressure.

2. The brake system of claim 1 wherein said control signal is a functionof the front wheel turning at the slower velocity.

3. A braking system for a wheeled vehicle having antiskid control, saidsystem comprising:

a brake master cylinder for generating pressurized fluid in response toa brake application;

actuating means for applying the brakes of said vehicle braking systemin response to said pressurized fluid from said brake master cylinder;

sensor means for detecting the rotational velocity of the wheels of saidwheeled vehicle, said sensor means generating a signal proportional tosaid rotational velocity;

control means for converting said proportional signal to controlsignals, said control signals being generated when there is an imminentskid condition;

modulator means for removing operator control and varying saidpressurized fluid received by said actuation means from said brakemaster cylinder in response to said control signals being received fromsaid control means;

valve means operatively connected to said modulator means wherein saidcontrol signals operate said valve means to give multiple pressure buildand decay rates in said actuation means; and I proportioning meansinterposed between said modulator means and said actuating means ofcertain wheels of said wheeled vehicle, said proportioning meansautomatically redistributing braking effort between said certain wheelsand the rest of the wheels of said wheeled vehicle as a function of loadshift.

4. The braking system having antiskid control, as recited in claim 3,wherein said valve means comprises solenoid valves located at a fluidinput and a fluid output of said modulator means, said control signalsoperating said solenoid valves to give said multiple pressure build anddecay rates.

5. The braking system having antiskid control, as recited in claim 4,wherein said control signals are generated when predetermined rates ofchange of said rotational velocity have been detected by said controlmeans, said control signals being repeated as long as said predeterminedrates of change occur on subsequent cycles of the same brakeapplication.

1. In a wheeled vehicle with front and rear wheels having a pressureoperated brake system, said brake system comprising: a booster typebrake unit; separate actuation means for each wheel of said vehicle,said actuation means being responsive to pressure from said booster typebrake unit; sensor means for detecting the rotational velocity of atleast one wheel of said wheeled vehicle and generating a sensor outputsignal for each detected wheel; control means for converting said sensoroutput signals into a control signal; modulation means interposedbetween said booster type brake unit and said separate actuation means,said modulation means being operated by said control signal to reducebrake pressure and prevent wheel skidding; proportioning meansinterposed between said modulation means and said separation actuationmeans for the rear wheels, said proportioning means automaticallyredistributing braking effort between the front and rear wheels as afunction of load shift; and said modulation means having valve means tooperate said modulation means in response to said control signal, saidcontrol signal operating said valve means to provide multiple buildrates in said brake pressure.
 2. The brake system of claim 1 whereinsaid control signal is a function of the front wheel turning at theslower velocity.
 3. A braking system for a wheeled vehicle havingantiskid control, said system comprising: a brake master cylinder forgenerating pressurized fluid in response to a brake application;actuating means for applying the brakes of said vehicle braking systemin response to said pressurized fluid from said brake master cylinder;sensor means for detecting the rotational velocity of the wheels of saidwheeled vehicle, said sensor means generating a signal proportional tosaid rotational velocity; control means for converting said proportionalsignal to control signals, said control signals being generated whenthere is an imminent skid condition; modulator means for removingoperator control and varying said pressurized fluid received by saidactuation means from said brake master cylinder in response to saidcontrol signals being received from said control means; valve meansoperatively connected to said modulator means wherein said controlsignals operate said valve means to give multiple pressure build anddecay rates in said actuation means; and proportioning means interposedbetween said modulator means and said actuating means of certain wheelsof said wheeled vehicle, said proportioning means automaticallyredistributing braking effort between said certain wheels and the restof the wheels of Said wheeled vehicle as a function of load shift. 4.The braking system having antiskid control, as recited in claim 3,wherein said valve means comprises solenoid valves located at a fluidinput and a fluid output of said modulator means, said control signalsoperating said solenoid valves to give said multiple pressure build anddecay rates.
 5. The braking system having antiskid control, as recitedin claim 4, wherein said control signals are generated whenpredetermined rates of change of said rotational velocity have beendetected by said control means, said control signals being repeated aslong as said predetermined rates of change occur on subsequent cycles ofthe same brake application.